runtimevar/hashivault: add HashiVault runtimevar driver

[JEETDESAI25]

Fixes #3611

• add runtimevar/hashivault module implementing a Vault-backed watcher (KV v1/v2, mount option, SecretError, local go.mod)
• add unit + conformance tests (requires runtimevar/hashivault/localvault.sh)
• add examples and docs (how-to section, stub page, goexample entries)
• wire the new module into allmodules, internal/testing/alldeps, and samples/gocdk-runtimevar
• tests (go test ./runtimevar/hashivault) pass with the dev Vault running.

[google-cla]

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[vangent]

Thanks, looks great!